The present invention relates to a cathode for use in an electron tube and a process of manufacturing the same.
The cathode for electron tube in a cathode ray tube is manufactured in such a manner that a tricarbonate containing barium as the principal ingredient is contained in a metal cup as of nickel with a transverse plate, anf then, it is subjected to an oxidation process.
Meanwhile, the resolution for the image of a cathode ray tube depends on the size of the diameter of the electron beams emmitted from the electron gun, and therefore, the smaller the diameter of the electron beams, the clearer the image on the screen. However, if a clear image is to be produced, the current density of the cathode should be very high.
However, the current density released through the cathode of the conventional tube is no more than 1 A square centimeter, this level of current density being far insufficient to obtain a high resolution.
Coming recently, efforts have been focused on high quality televisions, and to meet this trend, several methods have been proposed.
A typical example of them is illustrated in FIG. 4A, which consists of two types: one of them being a dispenser type constituted such that barium carbonate 12 filled into a cup 11 as of molydenum cup A is clad with a porous metal layer 13 in such a manner that thermal electrons should be released through the porous metal layer 13; and another one of them being an impregnation type constituted such that only a porous metal layer 13 is filled into said cup 11, and this is impregnated with a compound containing barium oxide (BaO), aluminum oxide (Al2O3) and calcium oxide (CaO).
Another example is illustrated in FIG. 4B, which is constituted such that an impregnated metal layer 14 formed by impregnating an impression compound into the porous metal layer 13 is disposed in the cup 11, and a porous metal layer 13 is clad upon the top of it, thus formimg a space division. In this type, there arises the problem that it is difficult to control the thickness of the impregnated metal layer 14 when fabricated and therefore, this has not been commercialized yet.
The improved cathodes as above described have the advantage that, when the current density is increased with regard to the heating of the carbonate, the evaporation loss of the barium due to the joule heating can be inhibited, thereby assuring a sufficient life expectancy of the cathode. However, due to the fact that pores are formed in the upper porous metal layer, the initial activation period is extended, that is, the time until the thermal electron release materials are diffused from the lower portion to the upper portion is extended, thereby giving the disadvantage that a quick start-up characteristics is deficient. Further, the porous metal layer contains usually tungsten as the principal ingredient, and therefore, if thermal electron release materials such as barium oxide by product, then there arises the problem that a reaction by-product defined by the following formula is created; EQU W +3Ba3Al2O6 +6CaO--3Ba2CaAl2O6+3Ba+Ca3WO6 (1)
The reaction by-product can be filled in the pores of the porous metal layer, and through the accumulation of this phenomenon, the diffusion velocity of the thermal electrons can be more and more lowered.
Therefore, according to the most recent trend, the efforts are focused on cladding iridium, osmium. rhenium and the like on the porous metal layer, or developing impregnation compound, but no satisfactory result has been seen yet.